Layered and Low-Dimensional Lead, Silver, and Bismuth Halide Perovskites Directed by Halogen-Substituted Spacer Cations

Hybrid organic–inorganic metal halides provide a diverse parameter space in which the optoelectronic properties can be tuned through the composition. The compositional tunability extends to the metal site, which can be expanded from single valent metals (e.g., Pb2+) to multivalent metals (e.g., Ag+ and Bi3+), and the dimension (2D, 1D, or 0D). However, a deeper understanding of how the organic cations template these metal halide structures is needed. Here, we synthesize and study the structures of a series of new layered and low-dimensional metal (Pb, Ag, and Bi) halides templated by the halogenated aryl spacer cations 2-chlorobenzylammonium (2ClBZ) and 3-chloro-2-fluorobenzylammonium (3Cl2FBZ). We report new lead perovskites, (3Cl2FBZ)2PbBr4, (2ClBZ)3PbI5, and (3Cl2FBZ)2PbI4, and compare them to their silver and/or bismuth analogs (2ClBZ)4AgBiBr8, (3Cl2FBZ)4AgBiBr8, (2ClBZ)3Bi2I9, and (3Cl2FBZ)4Bi2I10. In all structures, the halogen-substituted cations result in 2D or "pseudo-2D" layering, but the different halogen substituents introduce different distortions (tilting, octahedral distortion) and dimensional reduction to 1D or 0D depending on the metal and halide compositions. Optical absorption measurements reveal the bandgaps are tunable through metal sites, dimension, and cations to different extents. Furthermore, the 1D (3Cl2FBZ)4Bi2I10 crystallizes in the noncentrosymmetric space group Cmc21 and exhibits second-harmonic generation (SHG). The organic–inorganic interactions and resultant structural distortions examined here provide insights toward the engineering of noncentrosymmetry and dimensional control in hybrid metal halide perovskites.